The construction of the detector portion of an infrared analyzer disclosed in Japanese Laid-Open Patent Application No. 90983/1977 is shown in FIG. 8 as an example of the prior art. In this device a front light receiver 20 and a rear light receiver 21 respectively enclose a control gas, which can be a gaseous component of a material contained in a sample to be analyzed or a known gas which absorbs an infrared spectra very similar to that absorbed by the gaseous sample to be analyzed, and the two receiver chambers 20 and 21 are connected to a condenser microphone detector 22 in order to sense the differences in the pressure changes in the two chambers 20 and 21 due to the absorption of the infrared rays as a charge in capacity. A screening plate 23 is provided between the front receiver 20 and the rear receiver 21 and is freely movable in and out of the space between the two receivers. By shifting the position of the screening plate 23 into and out of the path of the infrared rays and thus regulating the quantity of infrared rays absorbed in the rear receiver chamber 21, an optical balance between the infrared absorptions in the two receiver chambers can be achieved, as shown in FIG. 9, wherein the curve a shows the change in the signal due to the absorption of infrared rays in the front receiver chamber and the curve b shows the change in the rear receiver chamber, the quantity of infrared absorption being along the Y axis and the position of the screening plate being along the X axis.
However, in this prior art device, the construction of the detector is very complicated and there are several troublesome problems in the manufacture of the device, since the chambers 20 and 21 must be made independently. Furthermore, the stability of the screening plate 23 or the stabilizing of the plate at any given position is a difficult task.
In order to overcome the disadvantages of the prior art, there was experimentally developed a detector for an infrared analyzer as shown in FIG. 1, using an optical adjusting system like that used in an infrared analyzer of the double beam type in the analyzer of the single beam type. That is, the detector portion shown in FIG. 1 has a front receiver chamber 1 and a rear receiver chamber 2 which respectively enclose a reference gas, i.e. a gaseous component of a material contained in the sample gas to be analyzed or a known gas which absorbs an infrared spectra very similar to that absorbed by the gas to be analyzed. Both chambers 1 and 2 are connected to a condenser microphone detector 3, in order to detect the difference in pressure variations in the chambers 1 and 2 due to the absorption of infrared rays as a change in capacity (that is, a change of voltage in the detector 3).
In this infrared analyzer, as shown in FIGS. 1 and 2, a screening plate 4 was provided in front of the front receiver chamber 1 which was freely movable perpendicularly across the light path in order to adjust the quantities of light which enter the two receiver chambers 1 and 2. However, in this mechanism, since the screening plate 4 acts to control the amount of light admitted to the two receivers 1 and 2 in the same way, the signal curves a and b due to the absorption of infrared rays in the two chambers 1 and 2 were found to be almost parallel, as shown in FIG. 3, wherein the Y axis is the quantity of the absorbed infrared rays and the X axis is the position of the screening plate 4, and the curves a and b represent the change in the quantities of infrared rays absorbed respectively in the front receiver chamber 1 and the rear receiver chamber 2. Because of these characteristic curves, it was impossible to change the difference (b-a) effectively by movement of the screening plate 4 and accordingly, this analyzer could not be used to achieve an effective optical balance.